Synthetic Auxin Resistance in Kochia Available from the HRAC website: hracglobal.com Auxin Resistant Kochia Kochia is a widespread annual weed known for its adaptability. Evidence of triazine resistance was detected as early as 1976 and to the ALS herbicides in the 1980's. Populations of Kochia (resistant to dicamba and fluoxypyr were first identified in Montana, US and Nebraska, US in 1994. These Group O/4 herbicides are also known as Synthetic Auxins and some herbicides within this mechanism of action have been used widely to control kochia. Since the initial discovery, other populations have been identified in the US states of Colorado, Idaho, Montana, North Dakota, and Nebraska. Levels of Resistance and Cross-Resistance Levels of resistance to synthetic auxin herbicides in kochia are relatively low, normally less than 5 fold, although artificially selected inbred lines have exhibited up to a 30 fold increase. Resistant plants are injured by synthetic auxins but recover, whereas susceptible plants die. Dicamba has been one of the more popular auxin herbicides used to control kochia. Therefore auxin-resistant biotypes are often identified in fields with a history of dicamba use and dicamba is the herbicide most often used to study these biotypes. However, these populations also can demonstrate cross-resistance to other auxin herbicides such as fluroxypyr. Pre- emergence applications of soil-active auxins, such as dicamba, may suppress auxin-resistant kochia biotypes, though the practice would likely select for higher resistance levels and is not recommended as a control measure. Mechanism of Resistance Auxinic herbicides mimic the action of the phytohormone IAA and rapidly induce a wide variety of genetic and physiological responses by binding to IAA receptor(s). Plants die from un-controlled cell proliferation. Studies found no difference between R and S populations in herbicide absorption, translocation or metabolism of dicamba and the mechanism of resistance is not well understood. Dicamba resistance in kochia is conferred by a single dominant gene. Mechanisms and inheritance with fluroxypyr in these biotypes has not yet been studied. Rate of Spread Compared to the rate of spread of ALS inhibitor and Glyphosate resistant kochia the dicamba and/or fluroxypyr resistant populations are unusually slow at spreading. Synthetic auxin resistant kochia is present in less than 1% of fields in the USA, possibly due to some yet undiscovered fitness penalty. Resistance to Other MOA’s Management of synthetic auxin resistant kochia is complicated by resistance to other herbicide mechanisms of action (MOA’s). Kochia has also evolved resistance to the following MOA’s in North America in agricultural fields, on railways and along roadsides. 1. PSII-inhibitors (TSR*) — first identified 1976, and now in the US states of CO, IA, IL, IN, KS, MT, ND, NE, WI, and WY. 2. ALS-inhibitors (TSR*) — first identified 1987, ALS inhibitor Kochia occurs in most of North America where kochia is a weed. 3. Glyphosate (NTSR* - amplified EPSPS) — first identified 2007 and it has been reported in the US states of CO, ID, KS, MT, ND, NE, OK, OR, SD, WY, as well as in the Canadian provinces of AB, MB, and SK. In 2012 it was estimated that 1/3 of fields in Kansas contained GR kochia. Multiple Resistance Kochia populations with resistance to more than one herbicide mechanism of action present growers with the biggest challenge. The most common combinations are ALS inhibitor resistance with resistance to glyphosate or dicamba. One population, from Kansas, exhibited resistance to PSII inhibitors, ALS inhibitors, EPSPS inhibitors and synthetic auxins. Dicamba — 0.125 lb ai/A | Photo: Dr. Bill Dyer R S * TSR = Target Site Resistance, NTSR = Non Target Site Resistance Additional information on herbicide resistance can be found at weedscience.org